Pulse width modulation, PWM, is used widely within the field of for instance audio amplifiers. Generally, the available pulse width modulation techniques may be categorized into typically three different types of modulation, natural pulse width modulation, NPWM, uniform pulse width modulation, UPWM or linearized pulse width modulation, hereinafter LPWM.
Generally, all pulse width modulation implies the technique of transforming or converting an input signal into an output square wave signal having a certain pulse width, at least partly defined by the input signal by comparison of the input signal to a reference signal.
A short review of the general understanding of the above-mentioned groups of pulse width modulation techniques will be given below.
Natural pulse width modulation, NPWM, typically implies the comparison of a continuous time signal, typically an analogue waveform signal, to a reference signal, typically a sawtooth signal. The output signals will then switch between typically two output levels when the input signal and the reference signal intersect.
The natural pulse width modulation technique, NPWM, is generally regarded as distortion free within the audio band.
Uniform pulse width modulation, UPWM, typically implies the comparison of a discrete time signal, typically a digital waveform signal such as a PCM signal, to a reference signal, typically a sawtooth signal. The output signals will then switch between typically two output levels when the input signal and the reference signal intersect. A well-known problem related to uniform pulse width modulation, UPWM, is that the input signal, due to its discrete nature, may basically not necessarily be represented at the time of intersection. This problem may be dealt with in different ways, e.g. simply by accepting the error and quantizing the intersection time according to a quantizing algorithm.
In order to counteract the inherent distortion, several PWM linearization techniques have been disclosed within the art.
Linearized pulse width modulation, LPWM, typically deals with emulation of the theoretical value of the input signal, if a sample of the input signal was actually present at the time of intersection between the reference signal and the input signal.
Such methods are often referred to as linearized pulse width modulation, LPWM. Prior art demonstrates linear interpolation between two adjacent input samples to achieve the output pulse width. In other words, the linearization algorithms typically operate on more than one sample of the input signal to determine the linearized output pulse width.
Thus several linearization techniques apply to first order interpolation of the input signal between the discrete samples in order to estimate the true cross point between the input signal and the reference signal.
Other techniques implying 2nd order interpolation have been applied for the purpose of coming closer to the “true” cross point, thereby minimizing the resultant harmonic distortion.
A problem of many of the relatively new and improved LPWM techniques is that the resulting improvement is relatively costly with respect to computing requirements. Thus, several linearized pulse width modulation techniques require division.
Typically, prior art LPWM-techniques either do not include an algorithm to determine the pulse width, or they disadvantageously use a division operation to compute the pulse width. Division operations are relatively computationally inefficient in digital signal processing and require many more computation steps than for example addition or multiplication operations. An additional silicon area is therefore required to implement techniques involving division. Generally known implementations do not provide computationally efficient methods to reduce harmonic distortion in pulse width modulated systems.
It is an object of the invention to provide a method, which may be applied in a PWM modulation technique featuring low distortion and computationally efficiency.